In recent years, developments in information technology have enabled the transmission of large quantities of recorded information. This has lead to demands for high-density, large-capacity optical discs that are capable of recording and reading large video or music files, or computer data or the like.
The DVD (Digital Versatile Disc), which is in widespread use as a high-density recording medium, has a structure in which two substrates of thickness 0.6 mm are bonded together using an adhesive. In order to enable higher density recording, the DVD uses a 650 nm laser that is of a shorter wavelength than that used for a CD (Compact Disc), and the optical system also has a higher numerical aperture.
However, in order to enable the recording and reading of high-definition video such as that associated with HDTV (High Definition Television), further increases in recording density are required. Investigations are already being conducted into methods of achieving even higher density recording, and into the optical discs required for such recording, which will act as the next generation of discs following DVD. High-density recording systems using novel optical disc structures that utilize a blue laser with an even shorter wavelength than that used for DVD and a high numerical aperture optical system have already been proposed (for example, Nikkei Electronics (Nikkei Business Publications, Inc.), 1999, Aug. 9, pp. 47 to 53.).
These new optical discs have a structure in which a recording layer is formed on top of a transparent or opaque substrate formed from a plastic such as a polycarbonate, and a light transmission layer of approximately 100 Mm is then created on top of the recording layer, wherein the recording light or reading light, or both types of light, are irradiated through the light transmission layer. For reasons of productivity, research of the light transmission layer of this type of optical disc has focused almost exclusively on the use of ultraviolet-curable compositions.
As an example of this type of optical disc in which recording or reading is performed using a blue laser, an optical disc has been proposed in which an information recording layer and a light reflection layer are formed on top of a circular substrate, and an ultraviolet-curable resin is then applied and cured to create a light transmission layer to the top of the disc. In this technique, a cationic polymerization-type ultraviolet-curable composition is used as the material for forming the light transmission layer (for example, see Japanese Unexamined Patent Application, First Publication No. Hei 11-191240 and Japanese Unexamined Patent Application, First Publication No. 2002-92948). However, cationic polymerization-type ultraviolet-curable compositions generate Lewis acids upon ultraviolet radiation that can cause corrosion of the light reflection layer, meaning achieving favorable stability for the optical disc upon long-term storage can be problematic. The material used for the light reflection layer requires a high reflectance at approximately 400 nm, and therefore silver or an alloy containing silver as the main component is typically used. Because silver and alloys containing silver as the main component are prone to chemical changes such as corrosion under the action of chemical agents, in those cases where silver or an alloy containing silver as the main component is used as the material for forming the light reflection layer, the use of a cationic polymerization-type ultraviolet-curable composition as the material for forming the light transmission layer that contacts the light reflection layer is undesirable.
An optical disc has also been proposed which, instead of using a cationic polymerization-type ultraviolet-curable composition, uses a typical radical polymerization system composed of an ultraviolet-curable composition containing a (meth)acrylate compound and a photopolymerization initiator as the main components (see Japanese Unexamined Patent Application, First Publication No. 2003-119231). This document discloses a radiation-curable composition with a low curing shrinkage ratio, low water absorption and excellent transparency, which contains urethane (meth)acrylate and tris(2-acryloyloxyethyl) isocyanurate, and is applicable to an optical disc in which recording or reading is performed using a blue laser. However, when this composition is used in an optical disc that uses silver or a silver alloy as the light reflection layer, corrosion tends to develop upon long-term storage under high temperature and high humidity conditions, and exposure to fluorescent light tends to cause a deterioration in the optical disc reflectance.
Although not proposed for a blue laser optical disc, another example of an ultraviolet-curable composition that has been proposed for use in a bonded optical disc is an ultraviolet-curable composition for bonding an optical disc that includes a reaction product between a half-ester compound, which is itself a reaction product of a (meth)acrylate containing one hydroxyl group within each molecule and a dibasic acid anhydride, and an epoxy resin having at least two epoxy groups within each molecule, as well as a reactive diluent and a photopolymerization initiator (see Japanese Unexamined Patent Application, First Publication No. 2003-206449 (“JP '449:)). This technique has an object of providing an adhesive composition that exhibits superior durability (reliability) when used in a bonded optical disc that contains a semi-transparent reflection layer formed from silver or the like, and the document reports that no change is observed in the external appearance of the reflection layer even after standing under high temperature and high humidity conditions. However, this technique does not relate to blue laser optical discs, but rather relates to bonded discs such as a DVD-9 disc.
Ultraviolet compositions suffer from a problem in that the shrinkage that occurs upon curing tends to cause warping of the substrate, but in a DVD structure in which the two substrates are bonded together using an adhesive layer formed from an ultraviolet-curable composition, the strain generated during curing of the ultraviolet-curable composition is suppressed by the rigidity of the substrates positioned on either side of the adhesive layer. In contrast, in a blue laser optical disc, although a substrate having a thick structure is formed on one surface of the light transmission layer, the other surface does not have a rigid substrate created thereto, and usually has only a comparatively thin layer such as a surface coating layer formed thereon. Furthermore, whereas the adhesive layer within a DVD has a thickness of approximately 50 μm, the light transmission layer of a blue laser optical disc is formed with an even greater thickness of approximately 100 μm. Accordingly, if an ultraviolet-curable composition used as an adhesive layer for a DVD is simply transferred to use within a blue laser optical disc, then a large degree of warping results.
Furthermore, in conventional CD and DVD discs, reading and writing of information is performed by transmission of light through a protective coating layer and an adhesive layer used for bonding, both of which are formed from ultraviolet-curable compositions. Because these layers composed of cured films of ultraviolet-curable compositions represent a thickness of only approximately 7 μm in a CD and approximately 50 μm in a DVD, even if some warping occurs, it has little adverse effect on the reading or writing of information. However, if these compositions are used for the light transmission layer of a blue laser optical disc, then because of the relatively thick film thickness, even comparatively small warping can cause read errors, and in the worst case scenario, may make reading impossible. As a result, in those cases where an ultraviolet-curable composition is used for the light transmission layer of a blue laser optical disc, warping tends to be a significant problem.
When an ultraviolet-curable composition containing the reaction product of 2-acryloyloxyethyl phthalate and a bisphenol A epoxy resin, which represents one of the examples disclosed in Patent Document 4, was used to form a light transmission layer within an actual blue laser optical disc, the warping was significant, and of a level that could not be ignored.